Outdoor Furnace Supply has 8"x8" water/air heat exchangers with 1" copper ports.

I would guess stacking about 4 of them would work

My HRV has 5" round ports

Plans have been approved, stamped, & I payed county.

I'm required to build what was approved.

DEnd, thanks for the links.

I read them a few months ago, good to refresh though.

Some reasons why I don't think I will have ice dams:

Bottom of roof sheathing, all rafters, & wall top plates will be completely encapsulated with R-50 closed cell SPF. No warm air from inside the house will get to the roof, no penetrations through roof, Radon & DWV vent pipes exit high up through the rear gable end wall. (Yes this meet code & is described therein)

This type of steel roof, sits up on horizontal 2x2 Batton's, with some air flow under them.

I never had ice dams on my last very similar house & climate zone.

The 600' of 1" pex will be 2 loops of 300' (as A/C said lower pumping energy)
2 manifolds using 1 ECM pump.

Pump will only run when HRV is on AND:
In Summer mode
OR
In Winter mode, when outside air temperature is colder than the loop water temperature.

Loops will be 7-9' below grade behind house & parking area retaining wall.
I expect the return temperature to be about 35*F in Winter & 45*F in Summer.

I will only add glycol if needed.

I will need to find an inline air filter placed before the air/water heat exchanger.

Since you are doing a custom install, maybe think about adding a second heat exchanger after the HRV, for summer use. The reason is you can up the efficiency of the HRV by running the hot outside air through it. This works because you will have a greater temperature difference between the intake and exhaust air streams. with the Heat exchanger after the HRV you can get the incoming air closer to the indoor air temperature or possibly even below it. The reason for tempering the air in the winter before it reaches the HRV is to help prevent core freeze up, and this isn't a problem in the summer.

DEnd, Yes that makes sense.

2 x water/air heat exchangers.
In Winter the DIY HX in the incoming cold fresh air before the HRV tempers the air warmer.
In Summer the DIY HX in the incoming hot fresh air after the HRV Tempers the air much cooler.

In the great room / kitchen (open concept) there will be much more heat loss in Winter compaired with the rest of house, this is due to the 8' x 42" entry door, a large sliding glass door 8' x 8', and 2 7' x 5' windows. I have decided to put down the 6x 185' 1/2" pex loops at 6" on center.

The 3 bedrooms will be 8" on center, the master bath will be its own zone / thermostat with a single pex loop. I'm tired of cold bathroom floors.

It would be nice to have warm floors in the bathroom, but what about running them under the tub at the same time? Stepping into the shower this morning, I thought about how nice it would be to have the shower floor warmed.

Geo, I will run the pex under the tub and under the shower.

I will keep the master bath warmer than the master bedroom, possibly 10-15*F or so.

The thermostats are Tekmar PWM type that learn about and correct overshoot & undershoot.

They have air & slab sensors, which I have never used before.

Micro zones like this can be a big problem with standard boiler installs & even mod-con boilers.

Think about this, the boiler must be sized to be able to keep every room at 70*F at "Design Temperature" (2*F here)

With multiple zones (4 this house) & say it is 50*F outside, then the smallest heat loss zone calls for heat, this could only a few % of the boilers required capacity, and any boiler would "short cycle" leading to lower efficency & early death.

My system has a buffer tank (79 gal.) between the heat source and the radiant zones.
My boiler will always have long efficient burn cycles
(79gal x 8.5# x 30* aquastat Delta) = 20,145 BTU's about 30 minutes minimum for my boiler, and possibly much longer when system is calling for heat.

Sounds like an impressive undertaking! What are you by trade?